(i) Field of the Invention
The present invention relates to an image forming apparatus such as a copier, printer or facsimile apparatus and, more particularly, to an improved lens mounting device of an optical housing of the image forming apparatus
(ii) Description of the Prior Art
Generally, an image forming apparatus is provided with a laser beam scanner as an image reading device. It is a common practice with the laser beam scanner to mount an fθ lens as on an optical element on an optical housing which constitutes a part of the laser beam scanner. Various technologies for mounting the lens on the optical housing have been proposed and employed in the conventional laser beam scanners. For example, it is known that the lens is mounted on the optical housing through an adhesive layer if the lens directly comes into contact with the optical housing.
Usually, the optical housing is constructed in a manner as to be sealed, but an ambient temperature around the optical housing intensively varies due to a change in operating conditions of the image forming apparatus, for example, a change in cooling condition in a continuous operation mode or a standby mode in the image forming apparatus. With such a change in ambient temperature, a temperature in the inside of the optical housing also gradually changes to the ambient temperature. A temperature of the optical housing itself intensively fluctuates as compared with a change in the inside temperature of the optical housing because the optical housing is directly exposed to the surrounding environment.
Therefore, an optical element such as an fθ lens which is mounted on the optical housing in a manner as to be partially therewith or partially in contiguity therewith through an adhesive is greatly affected by a change in temperature of the optical housing from the mounting portion and a temperature of the lens locally varies. In such a case, a local distortion occurs in the lens, thereby locally deteriorating the optical characteristics of the lens. As a result, there is a problem that a quality of an image corresponding to the deteriorated portion of the lens, especially a quality of an image which is continuous in a sub-scanning direction is lowered. This problem arises, in particular, when a mold resin is used as a material of the lens or when a material having relatively low specific heat is used for the optical housing.
On the other hand, when an operation of the image forming apparatus is finally stabilized, an ambient temperature around the optical housing, a temperature of the optical housing itself and an inside temperature of the optical housing are also stable. As a result, a local temperature gradient is not generated in the optical element such as an fθ lens, and an image with the excellent quality in the optical characteristics can be obtained. In other words, when an operation mode of the image forming apparatus is changed and an ambient temperature around the optical housing suddenly varies, such a problem arise in particular.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-open Publication No. 2001-194609, Japanese Patent Laid-open Publication No. 2001-305456, Japanese Patent No. 2918921 and Japanese Registered Utility Model No. 2535084.